A digital versatile disc (DVD) allows digital information to be recorded thereon at a recording density about six times higher than a compact disc (CD), and is known as an optical disc capable of storing large capacity data. Along with the recent increase of the information quantity to be recorded on optical discs, an optical disc having a larger recording capacity is desired.
In order to increase the recording capacity of an optical disc, the recording density of information needs to be increased. Specifically, the size of an optical spot formed by light which irradiates an optical disc for recording information to the optical disc or reproducing information recorded on the optical disc needs to be decreased. For realizing this, it is necessary to decrease the wavelength of laser light from a light source and to increase the numerical aperture (NA) of an objective lens.
For DVDs, a light source for emitting light having a wavelength of 660 nm and an objective lens having a numerical aperture (NA) of 0.6 are used. For, for example, Blu-ray discs (BD) having a larger recording capacity, blue laser light having a wavelength of 405 nm and an objective lens having an NA of 0.85 are used. Thus, a recording density five times higher than that of an existing DVD is achieved.
For an optical information apparatus for performing high density recording and reproduction using laser light having a short wavelength, such as blue laser light or the like, compatibility with existing optical discs can improve usefulness thereof as an apparatus and increase cost performance. Conventionally, an optical head device as described below is available for realizing an optical information apparatus capable of compatibly performing recording to, or reproduction from, a plurality of optical discs of different recording densities.
FIG. 17 shows an example of a structure of a conventional optical head device.
A light beam 210 emitted from a light source 201 is transmitted through a polarization anisotropic hologram 202. The light beam 201 is converted into parallel light by a collimator lens 203 and is changed into light of circular polarization by a ¼ wavelength plate 204. A first objective lens 205 converges the light beam 210 on an information recording layer of an optical disc 327.
For an optical disc 328 of a lower recording density than that of the optical disc 327, only an inner part of the light beam close to an optical axis thereof is converged by a second objective lens 250 having a smaller numerical aperture (as represented with the dotted line). The first and second objective lenses are mechanically exchanged in accordance with the type of the optical disc. Where a single objective lens is used, a variable aperture is used to change the numerical aperture for the optical disc.
The light of circular polarization which is reflected by the optical disc follows the same optical path in the opposite direction. During this process, the light is converted by the ¼ wavelength plate 204 into light of linear polarization of a direction perpendicular to the light emitted from the light source 201. Therefore, the light is diffracted by the polarization anisotropic hologram 202 to be incident on light detectors 263 and 266. Based on signals obtained from the light detectors, the objective lens is moved along the optical axis to perform focusing control.
When the effective diameter of the light beam is reduced in order to apply a smaller numerical aperture for a disc of a lower recording density, the diffracted light is made smaller as diffracted light 213 and 214 (dotted lines) on the light detectors shown in a bottom part of FIG. 17.
FIG. 18 shows a waveform of a focusing error signal obtained from a conventional optical head device. A focusing error signal 3281 is reduced in the inclination angle in the vicinity of the center of the graph; namely, the sensitivity thereof is decreased. In order to avoid such a decrease of the sensitivity in the vicinity of the center, as shown the bottom part of FIG. 17, the width of divided areas of the light detectors is partially changed and the signal to be used is also changed in accordance with the change of the numerical aperture. By changing the numerical aperture in this manner, a focusing error signal 3271 with a constant sensitivity can be obtained.    Patent Document 1: Japanese Laid-Open Patent Publication No. 2000-207769